In plants such as sewage treatment plants, septic tanks, wells etc., it occurs that solid matter or other contaminations such as socks, sanitary towels, paper etc. obstruct the pump of the plant, for example a submersible pump that is submerged in the basin/receptacle of the plant. When the impeller and the impeller seat are positioned at a fixed distance from each other, the pollutants are sometimes too large to pass through the pump. Large pieces of solid matter may in worst case cause the impeller to wedge, thus seriously damaging the pump. Such an unintentional shutdown is costly since it entails expensive, tedious and unplanned maintenance work.
European patent EP 1,899,609 discloses a pump comprising a pump chamber with a rotatable impeller, said impeller being suspended by a drive shaft, and an impeller seat/suction cover. The impeller is movable in the axial direction in relation to the impeller seat so that it may allow larger pieces of solid matter to pass through, pieces that otherwise would block the pump or wedge the impeller. The impeller has a cylinder-shaped recess in which the lower end of the drive shaft unit is received, and the impeller is displaceable in the axial direction between a lower and an upper position. The impeller also has an axially extending hole that connects the cylinder-shaped recess and the pump chamber so as to allow for introduction of a suitable tool in order to connect the impeller to the drive shaft.
When the pump is in operation said through-hole is provided with a plug/cover so as to prevent that the pumped liquid and pieces of solid matter enter the cylinder-shaped recess. Nonetheless, it sometimes occurs when pump is being serviced, e.g. due to readjustment of the axial gap present between the impeller and the impeller seat, that the importance of remounting said plug before the pump is restarted is overlooked. Once the pumped liquid enters the cylinder-shaped recess of the impeller, the constituting parts eventually corrode which, in turn, entails that the axial displaceability between the impeller and the drive shaft is adversely affected or becomes completely impossible. Moreover, pieces of solid matter may enter the cylinder-shaped recess which may mechanically prevent axial displacement of the impeller in relation to the drive shaft.
Furthermore, the interface between the impeller and the drive shaft unit has relatively small extension, i.e. radial abutment between the drive shaft unit and the cylinder-shaped recess, which entails that the impeller runs the risk of becoming tilted once subjected to an axially applied asymmetrical force.
It should also be mentioned that submersible pumps of the above kind are used to pump liquid from basins that are difficult to maintain and that pumps often operate for 12 or more hours daily. It is therefore utterly desirable to provide a pump with long working life.